There is currently a great deal of activity directed toward enhancing the fiber-coax networking infrastructures, such as those used by cable television and telephone companies and referred to herein as "broadband cable." The thrust is two-fold: enhancing the downstream capacity of the networks to support new services, and providing for significant upstream capacity for new interactive services, including telephony and data networking. Here, "upstream" refers to transmission from a station to a head-end (or central office), and "downstream" refers to transmission from a head-end to the stations.
Many current plans for providing upstream capability utilize the well known Time Division Multiple Access (TDMA) method of assigning bandwidth to stations that have invoked a signaling method to indicate to the head-end that they wish to transmit. However, existing versions of TDMA do not provide as much flexibility in the use of the available bandwidth as is desired. Also, TDMA allocates peak bandwidth for the entire duration of a call, and thus does not take advantage of the statistical nature of the traffic.
Existing packet data solutions also do not extend well to the broadband cable environment. For example, Carrier Sense Multiple Access/Collision Detection (CSMA/CD) as described in "Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications," published by the Institute of Electrical and Electronics Engineers, American National Standard ANSI/IEEE Std. 802.3-1985, is arranged so that stations listen to the channel for a period to determine that it is IDLE before transmitting. This type of arrangement is very inefficient in the cable environment, because of the distances involved and the fact that the shared upstream channel cannot be "heard" by all subscribers. The propagation delay, and corresponding dead time of the channel, is much greater in cable networks than in the Local Area Networks that are the standard CSMA/CD environment.
As another packet example, the slotted Aloha approach described by L. Roberts in "ALOHA Packet System With and Without Slots and Capture," Computer Communications Review, April 1975, specifies a system arranged so that time is divided into a series of exact slot periods. Any station that has data may transmit at any time. When collisions occur, stations back off and retransmit according to some randomized algorithm. This approach has also been enhanced with a reservation scheme in which once a station acquires a slot, it can keep successive slots by setting a header field; the station may also signal that it has finished transmitting by changing that field. See, for example, Lam, S. S. Packet Broadcast Networks--A performance analysis of the R-ALOHA Protocol, IEEE Transactions on Computers, Vol. C-29, No. 7, July 1980, 596-603. Here again, the problem is that in the coax/fiber infrastructure under consideration, it is difficult to allow stations to directly listen to each other. Also, the unique tree and branch structure of the cable network requires that a new approach be used.
Also very important is the capability of an access protocol to serve the needs of both Synchronous Transfer Mode (STM) and Asynchronous Transfer Mode (ATM) applications. Many new applications, such as high speed data and MPEG-2 video, are likely to be transported using ATM techniques from a home or office. At the same time, applications such as voice and video telephony are likely to continue being transported using STM, for reasons of delay, error sensitivity, etc.
A technique known as DQDB, described in "IEEE 802.6: Distributed Queue Dual Bus Access Method and Physical Layer Specifications," is applicable for networks spanning a metropolitan area. However, DQDB employs a dual-bus structure and requires a well-defined upstream-downstream relationship between adjacent stations. It is possible to design a system to fulfill these architectural requirements for tree and branch networks, but the scheme would be very complicated and the cost would be extremely high.
Recently, a variation of DQDB, called Extended Distributed Queuing Random Access Protocol (XDQRAP), has been proposed by C. Wu and G. Campbell, "Extended DQRAP: A Cable TV Protocol Functioning as a Distributed Switch," Proc. of 1st International Workshop on Community Networking, San Francisco, Jul. 13-14, 1994, pp. 191-198. One of the drawbacks of this proposal is that it does not support both STM and ATM techniques. In its present form, it does not accommodate bursts of various lengths for different payloads, as required for STM connections. Further, the physical layer overhead (i.e., guard time and preamble) is very high, due to the requirement that a fixed number of multiple request mini-slots be associated with each data slot.
Accordingly, there remains a strong need for an access protocol for broadband tree and branch networks that adapts to the changing demands of a mix of STM and ATM applications, and efficiently allocates bandwidth to a variety of bursty and isochronous traffic sources. As a result, a U.S. Pat. No. 5,570,355, issued Oct. 29, 1996, to Dail et al., entitled "Method and Apparatus Enabling Synchronous Transfer Mode and Packet Mode Access for Multiple Services on a Broadband Communications Network," and assigned to the same assignee as the present application, presented an access arrangement for dynamically altering a boundary between ATM-type traffic and STM-type traffic occurring in a frame based upon an ATM-oriented access protocol. This access protocol is also referred to herein as ADAPt.